U.S. Pat. No. 3,360,749 shows a basic SAW device using interdigital electrode transducers. Such a SAW device has a substrate with at least a surface layer of piezoelectric material and acoustic surface wave transducers disposed on the piezoelectric surface. The transducers convert between electrical signals and acoustic surface waves propagating on the piezoelectric surface. The transducers can convert from an input electrical signal to output acoustic surface waves or from input acoustic surface waves to an output electrical signal. In the very popular structure shown, there is a one-to-one correspondence between the impulse response of the transducer and its electrode geometry as pointed out in the invited paper of April, 1973 IEEE Transactions on Microwave Theory and Techniques entitled "Impulse Model Design of Acoustic Surface-Wave Filters" by the inventor of the present invention along with D. T. Bell, Jr. and R. C. Rosenfeld, Volume MTD-21, No. 4, Pages 162-175. The electrodes are spaced to specify each half cycle in the impulse response. The class of devices realized by this type of transducer is generally referred to as transversal filters, in that the surface acoustic wave makes a single transverse from the input transducer structure to the output transducer structure.
A second major category of SAW devices encompasses resonator devices which were first disclosed by the inventor of the present invention and others in a paper entitled "UHF Surface Acoustic Wave Resonators", 1974 Ultrasonic Symposium Proceedings, IEEE, Catalog No. 74 CHO 896-ISU and U.S. Pat. No. 3,886,504. The basic resonator device consists of two arrays of grating reflectors which are placed to form a surface acoustic wave resonant structure. Interdigital SAW transducers are normally placed inside of a cavity region of the resonator to couple energy into and out of the structure. Resonators are frequently referred to as recursive devices, in that a surface wave bounces back and forth many times. Resonators are generally usable for narrow bandpass filtering and for frequency control application in the VHF and UHF frequency range.
The grating arrays which form the distributed reflective structures for resonators can be metallic or dielectric strips deposited on the surface of the piezoelectric crystal, grooves etched into the surface of the crystal, impurities ion-implanted into the surface, or any other disturbance to the surface which can result in a surface acoustic wave reflection. The transducer electrodes in a normal SAW interdigital transducer can cause reflections. These reflections internal to a SAW transducer have been referred to as "second order" or "higher order" effects in the more traditional interdigital transducer device as pointed out in papers entitled "Second Order Effects in Surface Wave Devices", IEE Transactions on Sonics and Ultrasonics, Volume SU-19, No. 3, July, 1972, by the present inventor and others and "Fundamental-and-Harmonic-Frequency Surface-Model Analysis of Interdigital Transducers with Arbitrary Metalization Ratios and Pularity Sequences", IEEE Transactions on Microwave Theory and Technique, Volume MTD-23, No. 11, November, 1975, by W. Richard Smith and William F. Pedler. The interdigital reflections destroy the one-to-one correspondence between the transducer impulse response and the electrode geometry, and have thus been viewed as detrimental to the transversal-type device. Internal reflection effects in transversal filters have been eliminated by increasing the number of electrodes per acoustic wave length resulting in the split-electrode geometry which typically has four electrodes per wave length as pointed out in "Reflection of a Surface Wave from Three Types of ID Transducers", 1972 Ultrasonic Symposium Proceedings, IEEE, No. 72 CHO 708-8SU by de Vries, et al. and U.S. Pat. No. 3,727,155 to de Vries. In split electrode transducers, the reflections from one electrode of the transducer are cancelled by reflections from an adjacent transducer electrode. In a three electrode per wave length geometry shown in U.S. Pat. No. 3,686,518 to the present inventor and William S. Jones, similar cancellation of reflective waves occurs but only when reflections from three electrodes are considered.
In a resonator having first and second reflective grating structures on a piezoelectric surface, an acoustic surface input transducer and an acoustic wave output transducer between the reflective grating structures, the distance from the center drive of each of the transducers to the associated reflection center of the respective gratings is typically around 80 wavelengths of the center operating frequency. The ratio of the distance from the center of drive of each of the transducers to the associated reflection center of the respective grating to the distance between the two-grating centers is typically less than 0.2 to 0.3 in order to prevent unwanted longitudinal modes. It is also desirable to make the transducers as long as possible to increase the coupling. Increasing the transducers length increases the above said ratio with the result is that at some point the resonant condition is satisfied at more than one frequency in the reflection bandwidth. The result has been that only weak coupling has been possible for spurious free devices. Shortening the length of the transducers, and thus reducing the coupling, leads to increased insertion loss in some applications and makes wider bandwidth filters unattainable since the ratio of the distance from the center of drive of each of the transducers to the associated reflection center of the respective gratings to the distance between the drive centers controls the bandwidth achievable.
Another factor which has had an important impact on fabrication and design is the desire to achieve the highest possible device quality factor, known as the device Q. It has been found that one of the more important loss mechanisms in SAW resonators is scattering into bulk modes which takes place at the point where the gratings terminate inside the cavity. Also, similar effects are obtained from scattering from the transducer electrode fingers. One of the more popular methods for removing this loss has been the so called "buried transducer" configuration, in which the electrodes of the transducers are themselves buried in grooves etched in the crystal surface as described in an article by William J. Tanski entitled "Developments in Resonators on Quartz" in the 1977 Ultrasonics Symposium Proceedings, IEEE. While this technique reduces a significant loss mechanism, it also increases the fabrication sensitivity of the device. It is found that in this case the absolute placement accuracy of the first photo mask relative to the second photo mask must typically be of the order of 1% of the surface wave length if accurate frequency reproducibility is to be achieved. Good resonance behavior can be achieved for larger placement inaccuracy than this, but the required frequency tracking accuracy cannot be achieved with poorer fabrication accuracy. At 200 megahertz, this implies a relative mask alignment accuracy of approximately 1500 angstroms. Weighted gratings achieved by tapering the depth of the gratings can also be used to reduce bulk scattering as described in an article by William R. Shreve, et al entitled "Fabrication of SAW Resonators for Improved Long Term Aging" in the 1978 Ultrasonic Symposium Proceedings, IEEE.
Yet another problem found with prior art devices has been that of poor frequency control. A device Q of 10,000 implies frequency accuracy to one part in 10.sup.5 to 10.sup.6. In oscillators, frequency control is achieved to about 1% or less of the reciprocal Q. In filters, it is necessary to control relative frequency to roughly 2% to 10% of the filter bandwidth. Exact frequency is controlled by satisfying resonance conditions, i.e., round trip phase shift must be an exact multiple of 360 degrees. Problems arise from different propagation regions inside the cavity, e.g. grooved regions, transducer regions and open regions which have flat surface. All have the same velocity to the first order, but it is necessary to look for parts in 10.sup.6. As a result, very tiny effects are devastating.